@Celestial D. Dragon 's idea sounds interesting. "Buggy becomes a Yonko after accidentally finding Uranus from Captain John's treasure map".
Hope that comes true. Him becoming a Yonko while Shichibukai just got disbanded, must have a strong reason. And not "have strong subordinates" again because that reason has been used to promote him into a Shichibukai.
Anyway,
@Celestial D. Dragon
Any explanation why Kaido apparently didn't have Pluton in his hands yet, although he has been ruling over Wano for 20 years?
His subordinates are here and there. Every location has been invaded and occupied by Kaido, he had 20 years searching all over the country, but why he didn't have Pluton yet?